1. Field of the Invention
The present invention it relates to an erect image finder, and more precisely, relates to a real image type of finder optical system.
2. Description of Related Art
In a real image type of erect image finder which uses a Porro prism, it is known to divide the Porro prism into two prism elements to increase the magnification of the finder. An image forming plane of an objective lens is formed between the divided prism elements. In this arrangement, an eyepiece having a short focal length can be used to observe the image formed on the image forming plane, so that the finder magnification can be increased.
By way of example, a real image type of finder in which a divided Porro prism is employed is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 63-226616. In this known finder, a first prism of the divided Porro prism has an incident surface of predetermined curvature. However, it is impossible to further increase the finder magnification and it is very difficult to shorten the overall length of the finder in the axial direction.
This is because, as best discerned from an analysis conducted by the inventors of the present invention, the emission surface of the first prism and the incident surface of the second prism are both planar in the optical system disclosed in Japanese Kokai No. 63-226616 mentioned above. Accordingly, if the prisms are located far from one another, it is necessary to increase the size of the second prism to receive all the light diffused by and emitted from the first prism. This leads to an increase in the focal length of the eyepiece, which makes it impossible to sufficiently increase the finder magnification. Consequently, the first and second prisms can not be separated from one another, and accordingly, the optical path can not be extended in the lateral direction and a reduction of the axial length of the finder can not be realized.